The invention relates to a method of binding emulsified resin and tar substances in circulating water of a wet-cleaning and wet precipitation system for waste gas produced in the wood industry.
Austrian Patent AT-PS E 87227 describes a wet precipitation method for exhaust air cleaning of a timber desiccation system in which the exhaust air is wetted with circulating water, then cooled to the dew point of 50-70.degree. C., whereupon the condensing aerosols are ionized by means of high voltage and finally precipitated in a wet precipitator. In this case, circulating water is used for all cleaning steps, and the method is performed without waste water, because the water balance is negative due to the evaporation cooling.
If the raw, unprocessed wood (e.g. pine) contains a large amount of resin and is desiccated at high temperatures (600-800.degree. C.), a quantity of steam-volatile resin, tar and fatty acids is stripped out in addition to the cellular water (wood moisture). These substances are still present as gases in the desiccation waste air at 120.degree. C. In the following cooling to the dew point of 50-70.degree., these substances condense as aerosols ("blue haze") and can therefore first be precipitated as a tar-like coating on the precipitation surfaces of the wet precipitator and subsequently washed from there with circulating water.
A property of the tar-like, resinous substances, however, is that they form stable emulsions with the water and therefore cannot be precipitated, or only poorly precipitated, with the installed centrifuges for removal of solids. Hence, these substance build up in the circulating water and, consequently, severe contamination and clogging occur.